Method for producing esters of multibasic acids

ABSTRACT

A process is described for preparing esters of an acid component selected from among polybasic C 4 -C 10  carboxylic acids and an alcohol component selected from among C 3 -C 12  alkanols or from among C 3 -C 12  alkanediols, the alkyl chain of which may have interruption by from 1 to 3 oxygen atoms, by a) heating at boiling point, in a reaction zone and in the presence of an esterification catalyst, a mixture essentially consisting of the acid component or of an anhydride thereof and of the alcohol component, b) using rectification to separate the vapor comprising alcohol and water into an alcohol-rich fraction and a water-rich fraction, and c) returning the alcohol-rich fraction into the reaction zone and conducting the water-rich fraction out of the process. The process is simple to carry out and permits rapid achievement of essentially quantitative conversion.

This application is a National Stage application of InternationalApplication No. PCT/EP01/13131, filed 13 Nov. 2001, which applicationclaims benefit of German Application No. DE 100 561 79.9, filed 13 Nov.2000.

The present invention relates to a process for preparing esters of anacid component selected from among polybasic C₄-C₁₀ carboxylic acids andan alcohol component selected from among C₃-C₁₂ alkanols or from amongC₃-C₁₂ alkanediols, the alkyl chain of which may have interruption byfrom 1 to 3 oxygen atoms.

Esters of the type mentioned are widely used as plasticizers inplastics, such as cellulose acetates, polyurethanes, PVC, polyacrylates,etc. They may be prepared by reacting the acid component or an anhydridethereof with the alcohol component in the presence of an esterificationcatalyst. The reaction is an equilibrium reaction. The equilibrium maybe shifted to the product side, i.e. the ester side, by continuousremoval of the water produced as by-product from the reaction. If thealcohol used has a region in which it is not miscible with water, it ispossible to distill off continuously from the reaction mixture a mixtureof the water of the reaction and alcohol, and, after phase separation,to return the organic phase to the esterification, while the aqueousphase is removed from the system. Since water is always soluble to someextent in the alcohol, water is also returned with the organic phase,and the shift of the equilibrium to give quantitative conversionsometimes therefore proceeds very slowly.

If ethylene glycol monobutyl ether is used, for example, the region ofimmiscibility with water exists only at from 43° C. to 133° C. (cf. G.Schneider and G. Wilke, Zeitschr. f. Phys. Chemie, NF, 20, 219 (1959)),and in this case, therefore, the phase separator has to be helddisadvantageously at a temperature between the two limiting points ofmixing. In addition, the alcohol phase as it returns still comprises39.7% by weight of water.

It is an object of the present invention to provide a process forpreparing the esters mentioned at the outset which is simple to carryout and within a short time leads to essentially quantitativeconversion.

We have found that this object is achieved by

-   a) heating at boiling point, in a reaction zone and in the presence    of an esterification catalyst, a mixture essentially consisting of    the acid component or of an anhydride thereof and of the alcohol    component,-   b) using rectification to separate the vapor comprising alcohol and    water into an alcohol-rich fraction and a water-rich fraction,-   c) returning the alcohol-rich fraction into the reaction zone and    conducting the water-rich fraction out of the process.

The acid component is a polybasic, in particular dibasic, carboxylicacid having from 4 to 10 carbon atoms. It may be aromatic or aliphatic.Examples of those which may be used are succinic acid, glutaric acid,adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid,hexahydro phthalic acid, hexahydro isophthalic acid or hexahydroterepthalic acid, and also phthalic acid, isophthalic acid andterephthalic acid. Of these, adipic acid and phthalic acid areparticularly preferred. The acid component may be used as such or in theform of an anhydride, e.g. of an intra- or intermolecular anhydride. Forexample, phthalic acid is usually used in the form of the industriallyavailable compound phthalic anhydride.

The alcohol component is a branched or straight-chain alkanol oralkanediol having from 3 to 12 carbon atoms, preferably from 3 to 8carbon atoms, and the alkyl chain of the alkanol or alkanediol may haveinterruption by from 1 to 3, in particular 1 or 2, oxygen atoms in etherbonding. If more than one oxygen atom is present, there are preferablyat least two carbon atoms separating these from one another. It is alsopossible to use mixtures of different alkanols or alkanediols or ofalkanols and alkanediols. Examples of alkanols which may be used aren-propanol, i-propanol, n-butanol, sec-butanol, tert-butanol, pentanol,hexanol, heptanol, octanol, isooctanol, 2-ethylhexanol and also ethyleneglycol monomethyl ether, ethylene glycol monoethyl ether and ethyleneglycol monobutyl ether. Examples of alkanediols which may be used are1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,neopentyl glycol and 1,6-hexanediol. Of these, particular preference isgiven to n-butanol, 2-ethylhexanol, ethylene glycol monoethyl ether andethylene glycol monobutyl ether.

The esters of the alkanediols obtained according to the presentinvention are generally polymeric compounds with weight-averagemolecular weights of from about 1800 to 13000. For producing thesepolymeric esters, the alkanediols are generally used in a mixture withalkanols, wherein the latter are used as chain-terminating agents.

It is preferable for the alcohol component to be used in a molar excessover the acid component, e.g. in an excess of up to 500%, preferably upto 200%, in particular up to 100%, particularly preferably from 20 to60%, based on the stoichiometrically necessary amount. The process ofthe invention is carried out in the absence of any external entrainer.

Esterification catalysts which may be used are conventional catalystsusually used for esterification reactions. These include mineral acids,such as sulfuric acid, phosphoric acid; organic sulfonic acids, such asmethanesulfonic acid, p-toluenesulfonic acid; Lewis acids, in particulartitanium, tin(IV) or zirconium compounds, such as tetraalkoxytitaniumcompounds, e.g. tetrabutoxytitanium, and tin(IV) oxide.

An effective amount of the esterification catalyst is used and isusually in the range from 0.05 to 5% by weight, preferably from 0.1 to1% by weight, based on the entirety of acid component (or anhydride) andalcohol component.

The mixture of acid component or the anhydride thereof, alcoholcomponent and esterification catalyst is heated at boiling point. Thepressure and temperature conditions required for this purpose are highlydependent on the alcohol component used. A general guide which may begiven is from 140 to 220° C. and a pressure of from 0.3 to 2 bar. It isoften advantageous to raise the temperature during the course of thereaction and/or to lower the pressure.

Any of the commonly used heatable reactors is suitable for carrying outthe process of the invention, e.g. a stirred tank reactor.

In an advantageous method, the vapor comprising alcohol and water isreturned into a column for separation by rectification, and the productbeing discharged from the lower end of the column, i.e. the bottomproduct, is returned into the reaction zone as alcohol-rich fraction,and the distillate is conducted out of the process.

The column may be a conventional column known to the skilled worker,such as a packed column or a plate column. The packed columns may be ofglass, ceramic material or metal, e.g. Raschig rings, saddles, beads,spirals, other rings, and the like. An example of a suitable number oftheoretical plates of the column is from 8 to 12. The lower end of thecolumn has a connection to the vapor space of the reaction zonepermitting the passage of gas. In an advantageous method, all of therectified vapor emerging at the upper end of the column is condensed.Some or all of the condensate is taken off as distillate. The portionnot taken off as distillate is returned into the column as runback. Aparticularly advantageous runback ratio R/D has proven to be from 1:2 to1:10. If the condensate forms both phases, it is useful to recycle theorganic phase completely and to withdraw the aqueous phase at least inpart.

For the purposes of the present invention, the terms “alcohol-rich”fraction and “water-rich” fraction mean that the respective fraction hasbeen enriched in alcohol or, respectively, enriched in water whencompared with the composition of the vapor drawn off from the reactionzone. The alcohol-rich fraction preferably has an alcohol content ofmore than 80% by weight, particularly more than 95% by weight. It isparticularly preferably essentially pure alcohol.

Once the desired conversion has been achieved, if the reaction is beingoperated batchwise it is generally stopped, and the excess alcoholcomponent is distilled off. The desired ester may be isolated from theresidue using conventional processes, e.g. extraction to remove thecatalyst or the hydrolysis products thereof from the residue, followedby distillation. As an alternative, the process of the invention may becarried out continuously, and an advantageous method for this purpose isto use an arrangement of main reaction zone and post reaction zone, or acascade of reaction zones. Here, the vapor from at least one of thereaction zones and preferably from the main reaction zone is separatedas described, the vapor from each of the reactors being rectified on itsown or combined and rectified jointly. The runback of the alcohol-richfraction takes place into the respective reaction zone, or the fractionis divided as required among each of the reaction zones.

In an advantageous method, the water-rich fraction is worked up in orderto reclaim the alcohol component present therein. It is often possibleto make use of the fact that—as is the case with ethylene glycolmonobutyl ether—the composition of the water-rich fraction lies withinthe region where the alcohol component is not miscible with water. Inthe case of ethylene glycol monobutyl ether the region of immiscibilityoccurs only at from 43° C. to 133° C. A suitable method is therefore tobring the water-rich fraction to a temperature within the rangementioned, preferably from about 60 to 90° C., in particular from 70 to80° C., whereupon this breaks down into an organic phase with anethylene glycol monobutyl ether content of about 60.3% by weight and anaqueous phase with about 90.4% by weight of water. The organic and/oraqueous phase may be further purified if desired by conventionalprocesses.

The examples below will now give further illustration of the invention.

EXAMPLE 1

444 g (3 mol) of phthalic anhydride and 1063.8 g (9 mol) of ethyleneglycol monobutyl ether are heated at boiling point withtetrabutoxytitanium (0.8% by weight, based on the mixture) in a 2 lthree-necked flask, on top of which has been placed a packed column oflength 60 cm with column head and condenser, and also a runback divider,until a significant level of runback had appeared in the runbackdivider. A portion of the condensate from the rectified vapor was thenremoved at a temperature of 99° C. After 120 minutes, a total of 75 mlof condensate had been taken off, and the acid value found ondetermining the acid content of the flask contents was 0.056 mg KOH/g.

COMPARATIVE EXAMPLE 1

Example 1 was repeated, but instead of the packed column a waterseparator had been placed on the flask, the phase separator of the waterseparator being a jacketed vessel. This was held at 88° C. by athermostat. The upper phase ran continuously back into the flask. After300 minutes the acid value of the flask content was 9.6, and after 540minutes it was 0.12.

EXAMPLE 2

451 g (3.1 mol) of adipic acid and 792 g (7.75 mol) of n-hexan-1-ol werecharged to a 2 l three-necked flask which had a stirrer and on which a50 cm column and column head had been placed. 0.5% of methanesulfonicacid was added as catalyst, and the mixture was heated at boiling point.The condensate broke down into an organic phase and an aqueous phase.The manner of operating the column head was such that all of the organicphase was used as runback, while the aqueous phase was used as runbackand also taken off at the column head. The column head temperature wasfrom 95° C. to 100° C. After 110 minutes the acid value of the flaskcontents was 0.35.

COMPARATIVE EXAMPLE 2

Example 2 was repeated, but the column was replaced by a waterseparator. All of the organic phase was used as runback, while theaqueous phase was drawn off. After 240 minutes the acid value in theflask contents was 0.7.

1. A process for preparing esters of an acid component from amongpolybasic C₄-C₁₀ carboxylic acids and an alcohol component selected fromamong ethylene glycol monobutyl ether, n-hexanol-1, and 2-ethylhexanolby a) heating at boiling point, in a reaction zone and in the presenceof an esterification catalyst, a mixture consisting essentially of theacid component or of an anhydride thereof and of the alcohol componentto obtain a vapor comprising alcohol and water, b) introducing the vaporcomprising alcohol and water into a rectification column and separatingthe vapor into an alcohol-rich fraction and a water-rich fraction, byrectification, c) returning the alcohol-rich fraction being dischargedfrom the lower end of the rectification column into the reaction zone,condensing all of the water-rich fraction emerging at the upper end ofthe column to form an organic phase and an aqueous phase, recycling theorganic phase completely as reflux, and withdrawing from the process atleast a part of the aqueous phase.
 2. A process as claimed in claim 1,in which the water-rich fraction taken off out of the process is workedup in order to recover the alcohol component therein.
 3. A process asclaimed in claim 1, in which the acid component has been selected fromamong phthalic acid and adipic acid.
 4. A process as claimed in claim 1,in which the esterification catalyst has been selected from amongmineral acids, organic sulfonic acids and Lewis acids.
 5. A process asclaimed in claim 1, in which the alcohol component is ethylene glycolmonobutyl ether and the anhydride component is phthalic anhydride.
 6. Aprocess as claimed in claim 1, in which the alcohol component isethylene glycol monobutyl ether and the acid component is adipic acid.